Archdall, Mervyn (1723–91), clergyman and antiquary, was born 22 April 1723 in Dublin, elder son among two sons and three daughters of William Archdall, goldsmith, and his wife Henrietta, a widow who was daughter of Henry Gonne, curate of Finglas. William Archdall was Dublin assay master from 1736 until his death in 1751, and was a member of the gentry family from Castle Archdale, Co. Fermanagh. Mervyn Archdall was educated by Dr Keenane, and entered TCD in 1739; he graduated BA (1744) and MA (1747), and was ordained in the Church of Ireland. He was curate of Howth (1750–53), and of Kilgobbin and Taney, also in Dublin (1753–8), rector of Nathlash in the diocese of Cloyne (1749–58), and from 1756 domestic chaplain to Richard Pococke (qv), bishop of Ossory. In 1761 Pococke gave him the livings of Agharney and Attanagh in Ossory whch Archdall held until 1786, when he became rector of Slane in the diocese of Meath; he was also prebendary of Cloneamary (1762–4) and of Mayne (1764–72), both in Ossory.
Archdall was interested, almost from his student days, in ancient history and antiquities, and for forty years he gathered material for a work similar to an earlier compilation, Dugdale's Monasticon Anglicanum. He intended to publish two or more large folio volumes, but after Pococke's death had no sponsor and had to pay for its publication himself and cut back the scale of the project. Monasticon Hibernicum appeared in 1786 as a quarto volume, though still of over 800 pages. Archdall attempted encyclopaedic coverage of the history of Ireland's pre-reformation monasteries and abbeys. The work was ground-breaking and ambitious, though marred by mistakes and inadequacies. Archdall received help from friends, including scholars such as Pococke and Edward Ledwich (qv), and with them and others was a founder member of the RIA in 1786. In 1789 he published his second important work, a new edition of the Peerage of John Lodge (qv). Lodge's original work in four volumes was expanded by Archdall into seven octavos, adding a good deal of material from Archdall's own research to update the genealogies, as well as incorporating manuscript additions unpublished by the original author. These had been left in a cipher or in a private form of shorthand which could not be decoded by any contemporary expert in Dublin, and Archdall was just about to abandon the attempt to read them when his wife worked on it and discovered the key. She was his second wife, Abigail Archdall (née Young), whom he had married 25 November 1782. His first wife, Sarah Colles or Collis (d. 28 May 1782), is said to have been a relative of Thomas Prior (qv), though she may also – or instead – have been connected in some way to Pococke; the bishop is known to have educated Christopher Colles (1738–1816), who was to become an important engineer and entrepreneur in the US. Archdall and Sarah Colles married in Dublin (30 July 1747) and had two sons and a daughter; the elder son died young.
Archdall's works are still of interest to historians and genealogists, and surviving copies are valuable collectors’ items. In an apologia in his preface to the Peerage he noted ‘I have left that inaccurate which could not be exact, and that imperfect which cannot be completed’. He died suddenly 6 August 1791, and was buried in Slane churchyard.